Conventionally, an ultrasonic sensor is widely used in various fields. For example, it is used as a vertical sonar in an echo sounding machine and a fish detector, as a horizontal sonar for a vessel detecting a direction or a distance to an object, and as an ultrasonic diagnostic equipment for making a diagnosis of an internal organ by imaging the organ.
In addition, developments have been made for using the ultrasonic sensor in an art for monitoring a vehicle periphery and thus contributing to driving safety by detecting a distance to or a two-dimensional or three-dimensional position of an object around the vehicle. To detect the distance or the position, the ultrasonic sensor incorporated in the vehicle transmits an ultrasonic signal which is harmless to humans and receives the ultrasonic signal reflected by the object. For example, a parking assistance system is in practical use for assisting a driver in backing the vehicle in a parking space while avoiding objects such as a human or an obstacle located at the rearward of the vehicle. In the parking assistance system, a device generally referred to as a rear sonar is used which has the ultrasonic sensor at a rear portion of the vehicle and detects the object by means of the ultrasonic sensor.
For example, in the 2nd to 4th pages and FIGS. 1–3 of JP-H5-347797-A, such an ultrasonic sensor (specifically ultrasound probe) used for detecting a position and a distance of a target object is disclosed. Specifically, the ultrasonic sensor in the publication includes a piezoelectric complex and an electrode array. The piezoelectric complex is made of multiple piezoelectric substances which are connected with each other through organic polymers, and the electrode array is made of multiple electrodes which are aligned with intervals among them on a surface of the piezoelectric complex. In addition, portions of the piezoelectric complex overlapping the intervals of the electrode array are filled with filling materials, the dumping factors of which are higher than those of the organic polymers.
In the conventional ultrasonic sensors including the ultrasonic sensor in the publication, a receiver for converting a received ultrasonic signal to an electric signal has a one-dimensional structure or a two-dimensional structure. The one-dimensional structure is constructed by aligning, orderly in a single direction on a plane, multiple vibration plates (which correspond to the electrode array in the publication) with the same size and shape. The two-dimensional structure is constructed by aligning, orderly in lengthwise and crosswise directions on a plane, multiple vibration plates with the same size and shape. In addition, a transmitter of the conventional ultrasonic sensor converts an externally received electric signal to an ultrasonic signal and transmits the ultrasonic signal. The receiver receives at the vibration plates the ultrasonic signal reflected by the target object.
Then, the receiver detects gaps among timings at each of which one of the vibration plates receives the reflected ultrasonic signal. By comparing the detected gaps with the transmitted ultrasonic signal, the ultrasonic sensor can determine the two-dimensional or three-dimensional position of the target object, the distance between the target object and the ultrasonic sensor, etc.
The transmitter of the conventional ultrasonic sensor ordinarily transmits the ultrasonic signal by using a single vibration plate (or sound generation plate). However, a transmitter has been proposed which transmits the ultrasonic signal by using multiple vibration plates, which has the one-dimensional structure or the two-dimensional structure. It is a purpose of the transmitter having the multiple vibration plates to increase transmission power of the ultrasonic signal.
Since the conventional receiver has the structure constructed by aligning the multiple vibration plates with the same size and shape, all vibration plates have the same resonance frequency which causes an acoustic crosstalk among the vibration plates to be larger. Therefore the conventional receiver has a problem in terms of accuracy in determining the position and the distance of the target object.
The ultrasonic sensor in JP-H5-347797-A reduces the acoustic crosstalk among the electrodes (or vibration plates) and improves its azimuth resolution, because the portions of the piezoelectric complex overlapping the intervals of the electrode array are filled with the filling materials having higher dumping factors than those of the organic polymers constituting the piezoelectric complex.
However, the ultrasonic sensor in the publication still has a problem that it requires a manufacturing process for injecting the filling materials, which causes a growth in its manufacturing cost. In addition, the ultrasonic sensor in the publication still has a problem that the volume and the weight of the filling materials cause a growth in the total physical size and the total weight of the ultrasonic sensor. Moreover, the ultrasonic sensor is required recently to transmit an ultrasonic chord in order to improve the accuracy in determining the position and the distance regarding the target object.